Bryan Singer wrote a treatment for X3 with his friends and when he got tired of being pushed around by Fox and Warner offered him the Superman franchise, he bolted. He did offer to produce the sequel and let Fox use his treatment, but they basically told him to get bent and then did everything they could to beat Superman Returns to the box office. Even Matthew Vaughn gave up, it was such a hopeless rush job.

As to Fantastic Four, who knows what Fox owns... but it ain't much. The deal they signed with the producer of the Corman version, which is who sold it to Fox, was so poor that they didn't get most of the characters associated with the franchise. Tim Story wanted to do Black Panther and then suddenly stopped talking about it, which is when I'm guessing the Fox lawyers realized they didn't have the rights to squat outside a small period of the comic. I mean, really, the best they could do after screwing up Galactus was Puppet Master?

If Fox inherited a deal where they only get access to the first few years of the comic, and they already blew the origin AND Galactus, they're better off selling it back to Marvel. At least with X-Men, they've got like 13,946 characters and 50 years of continuity.